Process for Separating Off Nitrogen from Natural Gas

ABSTRACT

The invention relates to a process for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, preferably natural gas, wherein:
     a) the feed fraction ( 1 ) is liquefied (E 1 , E 2 ),   b) is separated by rectification (T 1 ) into a nitrogen-enriched fraction ( 9 ), the methane content of which is a max. of 1% by volume, and a hydrocarbon-rich, nitrogen-depleted fraction ( 4 ),   c) the hydrocarbon-rich, nitrogen-depleted fraction ( 4 ) is subcooled (E 3 ) and expanded (b),   d) the expanded hydrocarbon-rich, nitrogen-depleted fraction ( 5 ) is separated (D 1 ) into a hydrocarbon-rich fraction ( 6 ), the nitrogen content of which is a max. of 1% by volume, and a nitrogen-rich fraction ( 7 ), and   e) the nitrogen-rich fraction ( 7 ) is added to the feed fraction ( 1 ).

SUMMARY OF THE INVENTION

The invention relates to a process for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, preferably natural gas.

Hydrocarbon-rich feed fractions or natural gases which contain nitrogen require suitable measures during their liquefaction in order to be able to limit the nitrogen concentration in the end product, liquefied natural gas (LNG), to 1% by volume. In the event of a higher nitrogen concentration, undesired and dangerous layerings occur within the LNG storage tank owing to differing densities. In order to avoid this, customarily nitrogen is removed from the process by withdrawing a nitrogen-rich fuel gas stream at the cold end of the liquefaction process. This fuel gas stream, compared with the feed fraction, has a significantly elevated nitrogen content. In this manner, the nitrogen content of the LNG product can be limited to a maximum of 1% by volume, even if the nitrogen concentration in the feed fraction to be liquefied is significantly greater than 1% by volume.

Liquefaction processes frequently comprise gas turbines which can use the abovementioned fuel gas stream at least in part. However, in this case, it must be noted that the maximum permissible nitrogen concentration of the fuel gas stream is between 20 and 40% by volume. If the nitrogen content of the feed fraction to be liquefied is so high that the maximum permissible nitrogen content of the LNG product and also of the abovementioned fuel gas stream cannot be met, customarily a highly concentrated nitrogen fraction having a methane content of less than 1% by volume is withdrawn from the liquefaction process; this highly concentrated nitrogen fraction can be released directly to the atmosphere. This nitrogen fraction can be generated by the separation of the fuel gas stream proceeding in what is termed a nitrogen-rejection unit, or generation of the nitrogen fraction can be integrated into the liquefaction process—in this case still before withdrawal of the fuel gas stream.

FIG. 1 shows a process of the prior art for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, in which the production of a highly concentrated nitrogen fraction is integrated into the liquefaction process.

Via line 101, a hydrocarbon-rich, nitrogen-containing feed fraction is fed to a liquefaction process shown by the heat exchangers or heat exchange zones E1 to E3. The circuit 120 drawn in dashed lines is in this case an arbitrary refrigeration process or an arbitrary refrigeration unit as can be used in the liquefaction and subcooling of the feed fraction.

In the heat exchanger or heat exchange zone E1, the feed fraction is first cooled. Then, it is fed via line 102 to a second heat exchanger or heat exchange zone E2 where the feed fraction is completely liquefied. Via line 103 and expansion valve a, the then liquefied feed fraction is delivered to a separation column T1. From the bottom of separation column T1, a hydrocarbon-rich, nitrogen-depleted fraction is withdrawn via line 104 and subcooled in the heat exchanger or heat exchange zone E3.

Via line 105, this subcooled fraction is withdrawn from the actual liquefaction process, expanded in valve b and fed to a separator D1. From the bottom of the separator D1, via line 106, the liquid LNG product fraction is withdrawn and fed to an LNG storage tank (not shown).

From the top of the separation column T1, via line 108, a highly concentrated nitrogen fraction is withdrawn; the nitrogen content thereof is customarily between 90 and 100% by volume. One part of this nitrogen fraction is released directly to the atmosphere via line 109, while a further substream of this nitrogen fraction, after passage through the reflux condenser E4 arranged in the separator D1, is applied via line 110 as reflux to the separation column T1.

However, a process as described with reference to FIG. 1 leads to problems if no consumer is present for the fuel gas stream 107 discharged from separator D1. In contrast to the highly concentrated nitrogen fraction 109, fuel gas stream 107 cannot be released directly into the atmosphere.

Thus, an aspect of the present invention is to provide a process of the type mentioned above for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, preferably natural gas, which avoids the described disadvantages and makes it possible, in particular, to release the total amount of the nitrogen contained in the feed fraction either together with the LNG product stream or together with the highly concentrated nitrogen fraction.

Upon further study of the specification and appended claims, further aspects and advantages of this invention will become apparent to those skilled in the art.

To achieve these aspects, according to the invention there is provided a process for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, in which

-   a) the feed fraction is liquefied, -   b) is separated by rectification into a nitrogen-enriched fraction,     the methane content of which is a max. of 1% by volume, and a     hydrocarbon-rich, nitrogen-depleted fraction, -   c) the hydrocarbon-rich, nitrogen-depleted fraction is subcooled and     expanded, -   d) the expanded hydrocarbon-rich, nitrogen-depleted fraction is     separated into a liquid hydrocarbon-rich fraction, the nitrogen     content of which is a max. of 1% by volume, and a nitrogen-rich     fraction, and -   e) the nitrogen-rich fraction is added to the feed fraction.

Further advantageous embodiments of the process according to the invention for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, which are subjects of dependent patent claims, are characterized in that

-   -   the nitrogen-rich fraction, before it is added to the feed         fraction, is compressed in a single-stage or multistage manner         and/or cooled and/or the nitrogen-rich fraction, after it is         added to the feed fraction, is compressed in a single-stage or         multistage manner,     -   provided that the feed fraction, before it is cooled, is         subjected to an adsorptive drying process, characterized in that         the nitrogen-rich fraction, before it is added to the feed         fraction, is used as regeneration gas for the adsorptive drying         process,     -   a substream of the cooled feed fraction is fed as stripping gas         to the subsequent separation by rectification of the liquefied         feed fraction,     -   the liquefaction and/or the subcooling of the hydrocarbon-rich,         nitrogen-containing feed fraction proceeds using any arbitrary         liquefaction process,     -   provided that the liquefied hydrocarbon-rich fraction, the         nitrogen content of which is a max. of 1% by volume, is stored         in a tank, the boil-off gas occurring in the tank is added to         the nitrogen-rich fraction and/or the feed fraction, and     -   at least a substream of the nitrogen-enriched fraction is used         for precooling the feed fraction.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and further details, such as features and attendant advantages, of the invention are explained in more detail below on the basis of the exemplary embodiments which are diagrammatically depicted in the drawings, and wherein:

FIG. 1 schematically illustrates an exemplary embodiment of the prior art; and

FIG. 2 illustrates an exemplary embodiment of the process according to the invention.

In contrast to a prior art process, as explained with reference to the procedure shown in FIG. 1, in accordance with the invention the fuel gas stream which occurs at the end of the liquefaction process and after the liquefied and subcooled feed fraction has been expanded is now no longer sent to a fuel gas consumer, such as a gas turbine for example, but instead is added to the feed fraction to be liquefied before it is fed into the cooling and liquefaction process.

The process according to the invention for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, and also further configurations thereof, will be described in more detail hereinafter with reference to the exemplary embodiment shown in FIG. 2.

In a similar manner to the procedure described in FIG. 1, the hydrocarbon-rich, nitrogen-containing feed fraction is fed via the lines 1 and 1′ to a first heat exchanger E1 and cooled therein. The feed fraction is then fed via the line 2 to a second heat exchanger E2 in which it is liquefied. Via line 3 and expansion valve a, the liquefied feed fraction is applied to the separation column T1.

Advantageously, a substream 2′ of the cooled feed fraction 2 is applied as stripping gas to the separation column T1 via the expansion valve c, whereby the rectification action of the separation column T1 is supported.

From the bottom of the separation column T1, via line 4, a hydrocarbon-rich, nitrogen-depleted fraction is withdrawn and subcooled in the heat exchanger or heat exchange zone E3.

The circuit 20 which is drawn as dashed lines is an arbitrary refrigeration process or an arbitrary refrigeration unit as can be used, for example, in the liquefaction and subcooling of the feed fraction.

Via line 5, this subcooled fraction is withdrawn from the actual liquefaction process, expanded in valve b and fed to a separator D1. From the bottom of the separator D1, via line 6, the liquid LNG product fraction is withdrawn and fed to an LNG storage tank (not shown).

From the top of the separation column T1, via line 9, a highly concentrated nitrogen fraction is withdrawn; the nitrogen content thereof is customarily between 90 and 100% by volume. One part of this nitrogen fraction is released directly into the atmosphere via line 10, while a further substream of this nitrogen fraction, after passage through the reflux condenser E4 arranged in the separator D1, is applied as reflux to the separation column T1 via line 11.

Advantageously, the reflux condenser E4 and also the separator D1 are arranged to be high enough that the reflux 11 can proceed to the separation column T1 by gravity without the use of a pump. In addition, the pressure in the separator D1 is at least 2 bar (absolute pressure), preferably 3 bar (absolute pressure), in order to make possible pump-free transfer of the LNG product fraction into an atmospheric LNG storage tank.

According to an advantageous configuration of the process according to the invention, at least one substream of the nitrogen-enriched fraction 10, before it is released into the atmosphere, can be used for precooling the feed fraction 1. If the feed fraction 1—as described hereinafter—is subjected to a drying process, the precooling is advantageously connected upstream of this drying process.

In further development of the process according to the invention for liquefying a hydrocarbon-rich, nitrogen-containing feed fraction, it is proposed that, provided that the LNG product fraction is stored in an LNG storage tank, the boil-off gas occurring in the tank is added to the nitrogen-rich fraction 7 and/or the feed fraction 1. For this purpose the boil-off gas from the atmospheric LNG storage tank is preferably first compressed to the pressure of the separator D1 and then, together with the nitrogen-rich fraction 7 withdrawn from the separator D1, compressed to the pressure of the feed fraction 1.

The nitrogen-rich fraction or fuel gas fraction withdrawn at the top of the separator D1 via line 7 is compressed C1 according to the invention in a single stage or multiple stages, cooled in the aftercooler E5 and then added via line 8 to the hydrocarbon-rich, nitrogen-containing feed fraction in the line 1. Provided that the feed fraction 1 is at a comparatively low pressure, the nitrogen-rich fraction 7 can first be mixed with the feed fraction 1 and then subjected together therewith to a compression.

Generally, the feed fraction 1, before it is fed into the liquefaction process, is subjected to a drying A, preferably an adsorptive drying process. Provided that this is the case, the nitrogen-rich fraction can be used as a regeneration gas in the adsorptive drying process A. After regeneration has been performed, the nitrogen-rich fraction can then instead be added to the feed fraction in the line 1.

Whereas in the prior art process procedure described with reference to FIG. 1, three different fractions—LNG product fraction, fuel gas fraction and highly concentrated nitrogen fraction—which are withdrawn from the process occur, in the case of the process according to the invention, there are only two fractions, that is to say the LNG product fraction 6 and also the highly concentrated nitrogen fraction 10 withdrawn at the top of the separation column T1.

Recycling the nitrogen-rich fraction 7 into the feed fraction 1 has the consequence that, advantageously, an open mixed cycle is superimposed on the liquefaction process. This open mixed cycle consists essentially of the components nitrogen and methane, and also small amounts of higher hydrocarbons and possibly oxygen and traces of helium. This open mixed cycle is precooled in the heat exchanger E1, completely liquefied in the heat exchanger E2 and in the separation column T1 is fractionated into a pure nitrogen fraction, the methane content of which is less than 1% by volume, and a methane-rich bottom fraction. The methane fraction of the open mixed cycle and the remaining amount of nitrogen which is not released at the top of the separation column T1 are subcooled together with the LNG in the heat exchanger E3, vaporized in the heat exchanger E4 for the reflux condensation of the separation column T1 and fed together with the gas phase from the separator D1 again to the feed fraction 1.

Together with the refrigeration process or the refrigeration unit 20, which can comprise all known techniques such as, for example, single-substance vaporization, mixture vaporization, work-performing fluid expansion and also any combinations thereof, there results a refrigeration cycle cascade having an open N₂/CH₄ mixed cycle at the cold end of the liquefaction process.

By the formation of the open mixed cycle, the reflux condenser E4 can be supplied with the required cooling independently of the composition and pressure of the feed fraction, in such a manner that the total amount of the nitrogen contained in the feed fraction is concentrated to the required purity and can be released as a highly concentrated nitrogen product stream. The release of an unwanted gas mixture which, in the prior art process, formed the fuel gas fraction is thereby avoided.

The entire disclosure[s] of all applications, patents and publications, cited herein and of corresponding German Application No. DE 10 2009 038458.8, filed Aug. 21, 2009, are incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. A process for liquefying a hydrocarbon-rich, nitrogen-containing, and methane-containing feed fraction, said process comprising: a) liquefying (E1, E2) said feed fraction (1), b) separating the liquefied feed fraction by rectification (T1) into a nitrogen-enriched fraction (9) having a methane content of at most 1% by volume, and a hydrocarbon-rich, nitrogen-depleted fraction (4), c) subcooling (E3) and expanding (b) said hydrocarbon-rich, nitrogen-depleted fraction (4), d) separating (D1) the expanded hydrocarbon-rich, nitrogen-depleted fraction (5) into a liquid hydrocarbon-rich fraction (6) having a nitrogen content of at most 1% by volume, and a nitrogen-rich fraction (7), and e) adding said nitrogen-rich fraction (7) to said feed fraction (1).
 2. The process according to claim 1, wherein said hydrocarbon-rich, nitrogen-containing feed fraction is natural gas.
 3. The process according to claim 1, wherein said nitrogen-rich fraction (7, 8), before being added to said feed fraction (1), is compressed (C1) in a single-stage or multistage manner and/or cooled (E5), and/or said nitrogen-rich fraction, after being added to said feed fraction (1), is compressed in a single-stage or multistage manner.
 4. The process according to claim 1, wherein said feed fraction (1), before being cooled (E1) for liquefaction, is subjected to an adsorptive drying process (A), and said nitrogen-rich fraction (7), before being added to the feed fraction (1), is used as regeneration gas for said adsorptive drying process (A).
 5. The process according to claim 1, wherein a substream (2′) of the cooled feed fraction (2) is introduced as a stripping gas into the separation by rectification (T1) of the liquefied feed fraction (3).
 6. The process according to claim 1, wherein the liquefaction and/or the subcooling of the hydrocarbon-rich, nitrogen-containing feed fraction proceeds using any arbitrary liquefaction process.
 7. The process according to claim 1, wherein said liquid hydrocarbon-rich fraction (6) having a nitrogen content of at most 1% by volume is stored in a tank, and boil-off gas occurring in said tank is added to said nitrogen-rich fraction (7) and/or said feed fraction (1).
 8. The process according to claim 1, wherein at least a substream of said nitrogen-enriched fraction (9) is used for precooling said feed fraction (1).
 9. The process according to claim 1, wherein said feed fraction (1) is liquefied by being cooled in at least a first heat exchanger and a second heat exchanger.
 10. The process according to claim 9, wherein after said feed fraction (1) is cooled in said first heat exchanger, a substream (2′) of the cooled feed fraction (2) is introduced as a stripping gas into the separation by rectification (T1) of the liquefied feed fraction (3).
 11. The process according to claim 10, wherein said substream (2′) is expanded before being introduced as a stripping gas into the separation by rectification (T1) of the liquefied feed fraction (3).
 12. The process according to claim 3, wherein said nitrogen-rich fraction (7, 8), before being added to said feed fraction (1), is compressed (C1) in a single-stage or multistage manner and/or cooled (E5).
 13. The process according to claim 3, wherein said nitrogen-rich fraction, after being added to said feed fraction (1), is compressed in a single-stage or multistage manner.
 14. The process according to claim 1, wherein, before being added to said feed fraction (1), said nitrogen-rich fraction (7) is compressed and cooled.
 15. The process according to claim 1, wherein at least part of said nitrogen-enriched fraction (9) from said rectification (T1) is cooled in a reflux condenser (E4), arranged within a separator (D1), by heat exchange with said expanded hydrocarbon-rich, nitrogen-depleted fraction (5), and delivered to said rectification (T1) as reflux (11).
 16. The process according to claim 1, wherein at least one substream of said nitrogen-enriched fraction (10) is used to precool said feed fraction (1).
 17. The process according to claim 16, wherein said feed fraction (1), before being cooled (E1) for liquefaction, is subjected to an adsorptive drying process (A), and precooling of said feed fraction (1) by said at least one substream of said nitrogen-enriched fraction (10) is performed upstream of said adsorptive drying process.
 18. The process according to claim 7, wherein said boil-off gas occurring in said tank is added to said nitrogen-rich fraction (7), and is compressed to the pressure of said nitrogen-rich fraction (7) before being added thereto.
 19. The process according to claim 7, wherein said boil-off gas occurring in said tank is added to said feed fraction (1), and is compressed to the pressure of said feed fraction (1) before being added thereto. 